Corrugated board can be manufactured in many different widths and thicknesses. The thickness of the corrugated board is determined by the number of medians and liners in the board. First, corrugations or ridges are created in a median by passing the median through a corrugator. Then, an alternating series of liners and medians, with an adhesive between each layer, are brought together in a moving surface to a form a corrugated board of desired thickness. The moving surface passes through an assembly line that includes a hotplate section, where heat and pressure are applied to dry the board and set the adhesive, and a cooling section, where the corrugated board is cooled. The moving surface is then cut and scored to make corrugated board of different shapes and sizes for boxes and other items.
The hotplate section includes a heated platform, typically a series of steam chests, that heat the corrugated board to set the adhesive and to remove moisture from the medians and liners. An array of pressure applicators press the corrugated board against the heated platform to assist in moisture removal and heat transfer. The pressure applicators press the corrugated board against the steam chest to ensure adhesion across the entire width of the corrugated board to prevent blisters from forming in the corrugated board.
Because the steam chests tend to warp over time, usually with a sag in the middle, a rigid pressure applicator would crush the edges of the corrugated board and leave blisters in the middle of the board. Many machines are also configured to manufacture corrugated board of varying width. These machines should be capable of varying the pressure applied across the machine width because the edges of the corrugated board, which are only supported by adjacent corrugated board on one side, are easier to crush than the middle of the corrugated board. In addition, it may be desirable to vary the pressure in the cross-machine direction in response to variable moisture content in the board. Specifically, it may be advantageous to apply extra pressure to wetter areas of the board. Multi-foot pressure applicators have been developed for applying variable pressure across the width of the steam chest (i.e. in the cross-machine direction).
In a typical configuration, the hotplate section of a machine for manufacturing corrugated board includes 16 steam chests that are 7.3 feet (2.2 m) wide and extend in combination about 21 feet (6.5 m) in the direction of machine flow. A row of eight pressure applicators may overlie each steam chest in the cross-machine direction. This allows pressure to be applied over more steam chests for thicker corrugated board and at higher machine speeds. For example, pressure may be applied over only four steam chests (i.e., one group) for single-median corrugated board, over eight steam chests (i.e., two groups) for double-median corrugated board, and over all sixteen steam chests (i.e., four groups) for triple-median corrugated board. In addition, to increase the production output of thinner gauges of board, the machine speed may be increased and pressure may be applied over more steam chests. The hotplate section thus includes a grid of pressure applicators including rows of applicators in the cross-machine direction and columns of applicators in the direction of machine flow.
The conventional configuration described above has certain shortcomings when used to manufacture thick corrugated board, such as triple-median board. Namely, it is difficult to transfer heat from the steam chests all the way through to the top layers of the board. The thicker corrugated board therefore requires more time in the hotplate section to bring the temperature of the top layers of adhesive to the required setting temperature. It is also difficult to remove moisture from wet areas in the top layers, which can cause the board to warp as it dries. To counteract these problems, the speed of the board must be slowed considerably to ensure adequate moisture removal from the top layers of the board and adequate heating of the top layers of adhesive. This decrease in the speed of the assembly line decreases the production output and increases the cost of the thick corrugated board.
Moreover, the corrugator belt propels the top layer of the board through the hotplate section while the bottom layer is in sliding contact with the stationary top surface of the steam chests. The drag on the bottom side of the board tends to separate the bottom layer from the top layer. Therefore, if inadequate heat is applied to set the adhesive early in hotplate section, the top layer tend to slide in relation to the bottom layers, which smears the adhesive and forms a poorly bonded board known as "zipper board." And simply increasing the temperature of the steam chests does not solve the problem. This is because overheating the bottom layers of the board can over-dry the board and crystallize the adhesive, which also creates "zipper board."
Overcoming these problems requires moderate heat in the steam chests and further slowing of the assembly line to reduce the drag on the bottom of the board and set the adhesive without overheating the bottom layers of adhesive or separating the layers of the board. Manufacturing thick corrugated boards using a conventional pressure applicator and steam chest arrangement can therefore be quite costly, as an extreme decrease in the speed of the assembly line may be necessary. In a typical configuration, for example, the machine speed may have to be reduced from a preferred linear speed of 500 feet (15.2 m) per minute to 150 feet (4.6 m) per minute or less when manufacturing triple-median board.
Changing the thickness of the corrugated board while continuously running the assembly line is another objective when manufacturing corrugated board. By adding or removing adhering layers, the thickness of the corrugated board can be changed while the board continuously runs through the hotplate section. If the assembly line has to be stopped to accommodate changes in the thickness of the corrugated board, or to adjust the heat to be applied, then the assembly line must then be restarted, resulting in a reduction in the manufacturing output of corrugated board.
Changing the width of the corrugated board while continuously running the assembly line is another objective when manufacturing corrugated board. By changing the width of the medians and liners entering the hotplate section, the width of the manufactured corrugated board can be changed. In a conventional machine for manufacturing corrugated board, the heat applied to the different widths of corrugated board cannot be regulated in the cross-machine direction, resulting in excessive heat loss when manufacturing narrow board.
Conventional machines for manufacturing corrugated board are expensive pieces of equipment that may be restricted to specific physical parameters depending upon the configuration and availability of space in the manufacturing plant and the size and configuration of the existing machine. To improve the control response of an existing machine for manufacturing corrugated board, it is often desirable to retrofit the existing machine within the physical limitations imposed by the configuration of the manufacturing plant.
In summary, there is a need for an improved machine for manufacturing thick corrugated board, such as triple-median board. Specifically, there is a need to increase the rate of heat transfer and moisture evaporation, without decreasing the speed of the assembly line, when manufacturing thick corrugated board. There is a further need to retrofit conventional machines for manufacturing corrugated board to increase the rate of heat transfer and moisture evaporation, without decreasing the speed of the assembly line, when manufacturing thick corrugated board.